1. Field of the Invention
This invention relates to a gaseous fuel-fired heating appliance and venting systems for such appliances. More particularly, this invention relates to gaseous fuel-fired heating appliances, such as residential furnaces, having multiple venting options which are selectable at the time of installation of the appliance by the installer. In particular, the invention enables the installer to allow dilution air to enter and mix with combustion products from the heating appliance so as to adapt the appliance vent gas composition to venting systems constructed from a variety of materials.
2. Description of Prior Art
Conventional gas heating appliances, such as furnaces, boilers, and water heaters provide the user with safe, economical space and water heating, all the while requiring little maintenance over a relatively long appliance lifetime. These appliances typically use single wall galvanized vent connectors and either a masonry chimney or Type-B vent pipe to vent the flue gases generated by the combustion process during operation of the appliances. The American National Standards Institute (ANSI) categorizes gas appliances based on the pressure produced in a special test vent and the difference between the actual temperature and the dew point temperature of the flue gas.
A conventional Category I space heating appliance is one which has a vertical vent which operates under negative static vent pressure with a minimum of condensation. A Category I gas appliance has an annual fuel utilization efficiency range of about 78% to 83%. Moisture normally does not condense from the flue gas in Category I appliances because the actual flue gas temperature is generally higher than 140.degree. F. above its dew point temperature. Conventional draft hood equipped appliances are Category I appliances as well as most mid-efficiency, fan-assisted appliances. Mid-efficiency, fan assisted appliances differ from the conventional drafthood appliance by having an induced-draft blower to draw the combustion gases through the heat exchanger and deposit them into a vent. These appliances are classified as Category I appliances if the flue gas temperature is in the same range as the conventional Category I appliance, and if the induced-draft blower and vent system are designed to maintain a negative pressure in the vent. Venting systems for Category I appliances typically include Type-B vents, lined masonry chimneys, and single wall metal vents.
Category II appliances also operate with negative vent pressure. However, because the vent gas temperature is generally less than 140.degree. F. above its dew point temperature, corrosion of the vent is a problem requiring the designer to use corrosion resistant vents to exhaust the flue gases. As a result, there are few, if any, Category II gas appliances on the market.
Category III appliances operate with a positive vent pressure used with a vent gas temperature generally at least 140.degree. F. above its dew point temperature. The annual fuel utilization efficiency of these appliances is typically in the range of about 78% to 83%. Because the pressure in the vent is greater than the pressure of the surrounding atmosphere, these appliances require an airtight vent to prevent leakage of flue gases into the residence. An example of a Category III appliance is a mid-efficiency furnace that is vented horizontally through the side-wall of a residence. Venting systems for Category III appliances typically include high temperature plastic and single wall stainless steel metal vents.
Category IV gas heating appliances operate with a positive vent pressure and at a vent gas temperature less than 140.degree. F. above the dew point temperature. Category IV appliances are high efficiency, condensing units with an annual fuel utilization efficiency above 83%. Because the pressure in the vent exceeds that of the surrounding atmosphere and because condensation occurs in the vent, these appliances require an air tight, corrosion-resistant vent that is equipped for condensate disposal. Venting systems for Category IV appliances typically include polyvinylchloride ("PVC") or chlorinated polyvinylchloride ("CPVC") vents.
Accordingly, it can be seen that the category to which a particular appliance is assigned is important because it establishes the installation requirements of the venting system for the particular appliance. For example, as suggested hereinabove, a Category I appliance may utilize traditional venting materials such as Type-B vent pipe or a masonry chimney, while a Category IV appliance will require a vent system built from corrosion resistant materials.
The flue gases of gas heating appliances, such as furnaces and water heaters, contain a large amount of water vapor. Because the industry has moved to higher efficiency appliances, and, subsequently, to lower flue gas temperatures, condensation of water and corrosive substances from the flue gases onto vent system surfaces is a major design issue. Due to the consequences of condensate formation, the use of single wall metal vent connectors is severely limited by building codes and most masonry chimneys require relining before the new appliance may be installed. Converting to a Type-B connector from a single wall connector may cost the building owner up to $60, while relining a chimney to protect against condensation can cost from around $200 to $300. Furthermore, problems with Category III appliances using high temperature plastic vents have prompted some jurisdictions and some appliance manufacturers to prohibit the use of high temperature plastics. Alternative stainless steel vent systems are available at a cost in the range of about $100 to $300. Thus, it will be apparent that, in many cases, existing vents may be completely inadequate for new appliances and may discourage the building owner from installing gas appliances or require the building owner to undergo an expensive and time-consuming vent system replacement.
In an attempt to avoid these costs, several manufacturers have designed appliances with draft hoods that entrain dilution air into the vent. Entraining dilution air into the vent reduces the amount of condensation formed during operation, thereby reducing the number of installations which would require chimney relining. Unfortunately, this process also allows heated room air to escape in an uncontrolled fashion, both while the appliance is operating and while the appliance is idle. The escaping heat increases the heat load on the building, thereby increasing the energy costs associated with controlling the building temperature. Furthermore, typical draft hood equipped appliances are susceptible to backdrafting, a particularly troublesome problem in multi-story houses.
Currently, heating appliances such as residential furnaces and boilers are provided to a building owner based upon the type of vent system present in the building. As a result, manufacturers of these heating appliances are required to produce a multiplicity of different appliance models, which models must be cataloged and stored by the distributor and installer, in order to accommodate each of the possible vent systems.